In recent years there has been a growing interest in portable radios, such as two-way radios and cellular telephones. Often a portable radio user will have a need to establish radio communication while in a vehicle. The portable radio, when located inside the vehicle may not perform satisfactorily over long distances because either the portable antenna is insufficient or the vehicle body shields the portable antenna. Thus, the user must either purchase a separate mobile unit or use the portable radio with an external antenna through a mobile adapter.
Coupling losses associated with inserting the portable radio into the mobile adapter are a major concern in the design of any mobile adapter. Conventional methods of connecting a portable radio antenna to an external antenna often require a changeover switch which functions to disconnect or connect the internal and external antennas such that the two antennas are not connected at the same time. Switching between the two antennas, however, has a tendency to degrade the efficiency of the system due to the loss of the associated mechanical or electronic radio frequency (RF) switch. These switch devices and the required RF interface connector also can add significant cost and complexity to the portable subscriber unit.
Another disadvantage associated with many mobile adapters is that some form of power amplification, commonly known as a power "booster", is required to compensate for the coupling losses incurred when the portable radio is inserted into the mobile adapter. An antenna coupler configured to eliminate the need for additional power amplification would be a further advantage in terms of manufacturing costs and parts count.
An alternative method to directly connecting the portable unit's RF path to the mobile adapter via an RF switch is to employ an electromagnetic coupling device in the mobile adapter. The coupler transfers RF energy to and from the portable radio's antenna eliminating the need for a switch and RF connector in the portable subscriber unit.
In applications where the radio antenna cannot be retracted into the radio housing, it is often required to extend the size or length of the antenna coupler to insure the most efficient coupling to the antenna. As seen in prior art FIG. 1, a typical coupling assembly 10 is used where grounding generally is accomplished through direct connection with the ground on the radio 11. The coupling assembly 10 includes a coupler 12 and a ground connection 13 for attachment to the radio ground. A coupling stub 15 is used to couple RF energy from the radio antenna 17 to an external antenna 19.
For best coupling efficiency, the coupling element is usually placed in close proximity to the portable unit's antenna. A fixed antenna, as compared with a retractable antenna is physically cumbersome to meet this requirement and prior art designs had to compromise efficiency for a more ergonomic and compact design. Therefore, a design that does not require proximity along the entire radiating element while still providing high efficiency is most desired.
Accordingly, there exists a need to provide an improved antenna coupler with an electrically extended ground plane beyond that of the ground offered by the radio for use in a mobile adapter. The antenna coupler should minimize RF coupling losses without increasing the overall size of the coupling structure.